Computerized overcurrent detector

Abstract

An overcurrent detector realized by using a computer. The apparatus is provided with an overcurrent detector (5) which calculates the temperature rise θ(n) =K 1 P(n)+K 2 θ(n-1), based on a current (In) flowing in an electric machine, under the conditions that the machine has an electric resistance (r), the machine works in a known ambient temperature, the allowable temperature rise (θ(max)) of the machine is known, a first coefficient (K 1 ) and a second coefficient (K 2 ) are predetermined. The overcurrent detector (5) further compares the temperature rise (θ(n)) with a difference between the allowable temperature rise (θ(max)) of the machine and the ambient temperature at which the machine is used, and determines that the machine is in an overcurrent position, when the former exceeds the latter.

We claim: 
     
       1. An overcurrent detector, comprising: an electric current sensing circuit for determining an instantaneous value of electric current (In) flowing in an electric machine sampled at successive time periods;   first calculating means for receiving the instantaneous value from said electric current sensing circuit and for calculating heat (P(n)) generated in the electric machine based on the instantaneous value of the electric current (In) for each of the successive time periods and outputting the heat (P(n));   second calculating means for receiving the heat (P(n)) from said first calculating means and for calculating a temperature rise (θ(n)) at a preset time period(N) according to:   θ(n)=K.sub.1 P(n)+K.sub.2 θ(n-1)        where K 1  is a first coefficient,   K 2  is a second coefficient, and   θ(n-1) is a preceding temperature rise a preceding sampling period;     comparing means for comparing the temperature rise (θ(n)) received from said second calculating means to a predetermined maximum allowable temperature rise (θ(max)) to determine an overcurrent condition when the temperature rise (θ(n)) is greater than the predetermined maximum allowable temperature rise (θ(max)).   
     
     
       2. An overcurrent detector according to claim 1, wherein said first calculating means including multiplying means for multiplying a square of the instantaneous value of the electric current (In) by a resistance (r) of the electric machine. 
     
     
       3. An overcurrent detector according to claim 1, further comprising storing means for storing the preceding temperature rise (θ(n-1). 
     
     
       4. An overcurrent detector, comprising: an electric current sensing circuit for determining an instantaneous value of electric current (In) flowing in an electric machine sampled at successive time periods;   average current calculating means for receiving the instantaneous value from said electric current sensing circuit and for determining an average value of the electric current (In) flowing in the electric machine at the successive time periods;   first calculating means for receiving the average value from said average current calculating means and for calculating heat (P(n)) generated in the electric machine based on the average value of the electric current (In) for each of the successive time periods and outputting the heat (P(n));   second calculating means for receiving the heat (P(n)) from said first calculating means and for calculating a temperature rise (θ(n)) at a preset time period (N) according to:   θ(n)=K.sub.1 P(n)+K.sub.2 θ(n-1)        wherein K 1  is a first coefficient,   K 2  is a second coefficient, and   θ(n-1) is a preceding temperature rise in a preceding sampling period;     comparing means for comparing the temperature rise (θ(n)) received from said second calculating means to a predetermined maximum allowable temperature rise (θ(max)) to determine an overcurrent condition when the temperature rise (θ(n)) is greater than the predetermined maximum allowable temperature rise (θ(max)).   
     
     
       5. An overcurrent detector according to claim 4, wherein each of the successive time periods has a length, and   wherein the average value calculating means comprises:   dividing means for dividing each of the successive time periods by a number (m) to form a plurality of subdivisions; determining means for determining an instantaneous value of the electric current for each of the plurality of subdivisions; and   summing means for summing absolute values of each instantaneous value and dividing by (m) to derive the average value of the electric current.